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Old 08-02-2016, 01:42   #1
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Differing revs in fwd and reverse

Hi, my boat is behaving strangely and it could well be a slipping transmission but I want to be sure before we have to replace it

Perkins 4108 and a transmission which is unidentifiable.
  • In neutral it can happily rev past 25,000.
  • In reverse full throttle gives 19,400 revs and
  • fwd only 17,400 revs.
Last time we tried motoring we could only get 2.5 knots fwd out of it.

The prop is a J-Prop feathering prop which has adjustable pitch
My question is what could cause different engine speeds in fwd and reverse? Sipping transmission I guess but the speed is very consistent not wavering up and down.

Anyone?
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Old 08-02-2016, 04:08   #2
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Re: Differing revs in fwd and reverse

Can't help directly but usually the forward and reverse ratios are different which results in higher revs in reverse.

I also reckon your RPM is out by a factor of ten - they haven't built a Perkins yet that will go to 25,000 RPM.
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Old 08-02-2016, 05:19   #3
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Re: Differing revs in fwd and reverse

Yes, but only a max fwd speed of 2.5 kts indicates something is amiss
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Old 08-02-2016, 06:44   #4
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Re: Differing revs in fwd and reverse

Don't scrap your transmission yet!

A Perkins 4108 should achieve 4000 rpm in neutral; and in forward/reverse as well if the prop is sized and pitched correctly.

If the rpm you suggested are out by a factor of 10, that would make it:
In neutral 2500
In reverse 1940
fwd only 1740 revs.

Still you are coming up short.
I'm guessing your engine is not producing the full 51 horsepower it should be, as the numbers are indicating you are overpropped.
Is it smoking at all?



If its running well, it could be just the throttle cable not adjusted properly.


The numbers above then would make sense. It is common for forward gear ratio to be 10-15% lower than the reverse ratio on many transmissions, usually resulting in slower reverse speeds. If the engine is not producing enough hp (or torque) then it cannot turn the prop to get to full speed.


Things to check:


Your throttle linkage. Disconnect at the engine and see if you get more travel by moving by hand. (assuming the factor of 10 error)


Your rpm; perhaps use a handheld tachometer. And check the shaft rpm, so you will know your gear ratio.

Try to determine your transmission type. Post a picture on here and someone should be able to identify it.

And check your prop. Is it clean, or is it covered in barnacles.
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Old 08-02-2016, 06:54   #5
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Re: Differing revs in fwd and reverse

FWIW - I once experienced something very similar and it turned out the be the throttle cable that was at fault. I forget the details but something was slipping, so that full movement of the control in the cockpit was only giving about 30% throttle at the engine.

Edit: Just fully read Kokanee's post, and he already suggested this. Time for another coffee.
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Old 08-02-2016, 07:07   #6
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Re: Differing revs in fwd and reverse

Or the prop is just less efficient in reverse..
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Old 08-02-2016, 07:10   #7
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Re: Differing revs in fwd and reverse

Is the prop clean? Or the hull for that matter? It doesn't take much growth to load up the engine. As for different rpms between forward and reverse that's the norm with some engine controls. Reverse throttle is retarded somewhat, probably for reasons related to safety.

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Old 08-02-2016, 08:10   #8
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Re: Differing revs in fwd and reverse

Quote:
Originally Posted by Warby12 View Post
Last time we tried motoring we could only get 2.5 knots fwd out of it.


This tells me something is badly wrong, maybe extreme fouling?
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Old 08-02-2016, 08:22   #9
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Re: Differing revs in fwd and reverse

Fouling sounds like part of the problem but more so improper set up of the feathering prop.?
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Old 08-02-2016, 08:23   #10
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Re: Differing revs in fwd and reverse

Hi. Same symptoms happened to us once on a charter out of Tortola -- couldn't get past 1500 RPM, couldn't develop more than a few knots of boat speed under power. It turned out to be a loose drive belt, just like on a car engine. Easy to tighten. That possibility hasn't been mentioned yet in this thread AFAIK.

Mind you, I'm just a charterer, not an expert. (And I didn't do the fix myself.) Plus, this is my very first post.

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Old 08-02-2016, 09:35   #11
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Re: Differing revs in fwd and reverse

Throttle cable or incorrectly adjusted pitch (too much pitch) on J-Prop.

J Prop Details
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Old 08-02-2016, 09:52   #12
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Re: Differing revs in fwd and reverse

Since it won’t get to full revs in neutral, it is doubtful that it is just a prop problem or dirty hull. Although that may be contributing to the issue while in gear.

Start with the throttle linkage.
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Old 08-02-2016, 09:53   #13
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Re: Differing revs in fwd and reverse

Quote:
Originally Posted by Warby12 View Post
Hi, my boat is behaving strangely and it could well be a slipping transmission but I want to be sure before we have to replace it

Perkins 4108 and a transmission which is unidentifiable.
  • In neutral it can happily rev past 25,000.
  • In reverse full throttle gives 19,400 revs and
  • fwd only 17,400 revs.
Last time we tried motoring we could only get 2.5 knots fwd out of it.

The prop is a J-Prop feathering prop which has adjustable pitch
My question is what could cause different engine speeds in fwd and reverse? Sipping transmission I guess but the speed is very consistent not wavering up and down.

Anyone?
First, as others have mentioned, check your tach scale, as these numbers are too high.

Q1: Has the boat always done this since you've had it, or is this new?

Q2: If new, what has changed since it last worked properly?

Q3: If nothing has changed, how long has it been since it last worked properly?

Ramblin Rod
Marine Service Provider
About Sheen Marine
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Old 08-02-2016, 10:59   #14
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Re: Differing revs in fwd and reverse

Quote:
Originally Posted by Warby12 View Post
Hi, my boat is behaving strangely and it could well be a slipping transmission but I want to be sure before we have to replace it

Perkins 4108 and a transmission which is unidentifiable.
  • In neutral it can happily rev past 25,000.
  • In reverse full throttle gives 19,400 revs and
  • fwd only 17,400 revs.
Last time we tried motoring we could only get 2.5 knots fwd out of it.

The prop is a J-Prop feathering prop which has adjustable pitch
My question is what could cause different engine speeds in fwd and reverse? Sipping transmission I guess but the speed is very consistent not wavering up and down.

Anyone?
Hopefully you typed by an 0. I, not being an expert, don't believe you have a problem. Your prop is more efficient in forward thus loading your engine more. I would not worry about it. Don't be rapping your engine up to max anyway.
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Old 08-02-2016, 13:22   #15
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Re: Differing revs in fwd and reverse

Wow! Thanks all

So it is not abnormal to have different speed

Coming out of the water today for a new shaft seal and I will go all over this prop.
  • It may be over pitched, I will check. (thanks for that link Deblen!)
  • The prop can be setup for left and right turning shafts so need to check this too (ours is left).
  • The boat is new to us so I can't say if it's a new problem.
  • It definitely blows blue smoke when pushed hard (not black which is more likely with over pitching, yes?)
  • Hull moderately dirty but the prop itself was scraped clean last last week
  • I have just checked the throttle linkage and it's correct My wife actuated it from the cockpit and I watched on the engine and it reached the same point on the injection pump in neutral and in both gears.
  • I didn't push the revs all the way in neutral as I didn't know the safe limit, I will see if it hits 4000 today.
  • Yes I wrote the revs wrong by a factor of ten sorry.
If no luck, I'll look deeper. Blue smoke and low power might indicate worn rings?


I did report some issues with this in an older thread but that turned out to be dirty fuel making the engine race. identifying the transmission was impossible Identifying a Hurth (I think) Gearbox


Thanks enormously everyone. We want to go cruising in a few weeks and 2.5 knots max speed in flat water is just plain unsafe
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